feat: get duration from audio pool

[guneyozsan]

Description

In order to get the duration of the audio pooled, it needs to create a new player for the source, causing unnecessary file/source reads. This PR aims to conveniently get duration from an already created pool, without using extra resources.

Adds getDuration() method to AudioPool.

• When duration is requested the first time, it uses the pool to get or create an available player, similiar to start().
• Subsequent calls return the result immediately from cache, without using a player.

Checklist

• The title of my PR starts with a [Conventional Commit] prefix (fix:, feat:, refactor:,
  docs:, chore:, test:, ci: etc).
• I have read the [Contributor Guide] and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation and added dartdoc comments with ///, where necessary.
• I have updated/added relevant examples in [example].

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

Related Issues

Couldn't find any

[guneyozsan]

These two required tests fail, but I'm not sure if they are relevant to this PR:
https://github.com/bluefireteam/audioplayers/actions/runs/18434296690/job/52525732461?pr=1954
https://github.com/bluefireteam/audioplayers/actions/runs/18434296690/job/52525732471?pr=1954

Run net start audiosrv
The requested service has already been started.

[Gustl22]

Thank you for contributung! I currently work on a refactor to make some tests work again. I will have a look at your PR as soon as its resolved :)